Auto merge of #3016 - alexDarcy:doc_freebsd, r=micbou

Updating documentation for FreeBSd 11.x

# PR Prelude

Thank you for working on YCM! :)

**Please complete these steps and check these boxes (by putting an `x` inside
the brackets) _before_ filing your PR:**

- [ X] I have read and understood YCM's [CONTRIBUTING][cont] document.
- [X ] I have read and understood YCM's [CODE_OF_CONDUCT][code] document.
- [ X] I have included tests for the changes in my PR. If not, I have included a
  rationale for why I haven't.
- [ X] **I understand my PR may be closed if it becomes obvious I didn't
  actually perform all of these steps.**

# Why this change is necessary and useful

The instructions for FreeBSD were not up-to-date. This works for FreeBSD 11.x (the latest). As I've only changed the readme, no tests were run.

[cont]: https://github.com/Valloric/YouCompleteMe/blob/master/CONTRIBUTING.md
[code]: https://github.com/Valloric/YouCompleteMe/blob/master/CODE_OF_CONDUCT.md

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README.md

@@ -494,10 +494,9 @@ Make sure you have Vim 7.4.1578 with Python 2 or Python 3 support.
 OpenBSD 5.5 and later have a Vim that's recent enough. You can see the version of
 Vim installed by running `vim --version`.
 
-FreeBSD 10.x comes with clang compiler but not the libraries needed to install.
+For FreeBSD 11.x, the requirement is cmake:
 
-    pkg install llvm38 boost-all boost-python-libs clang38
-    export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/usr/local/llvm38/lib/
+    pkg install cmake
 
 Install YouCompleteMe with [Vundle][].
 
@@ -506,17 +505,20 @@ using Vundle and the ycm_core library APIs have changed (happens
 rarely), YCM will notify you to recompile it. You should then rerun the install
 process.
 
-Install dependencies and CMake: `sudo pkg_add llvm boost cmake`
-
 Compiling YCM **with** semantic support for C-family languages:
 
     cd ~/.vim/bundle/YouCompleteMe
-    ./install.py --clang-completer --system-libclang --system-boost
+    ./install.py --clang-completer
 
 Compiling YCM **without** semantic support for C-family languages:
 
     cd ~/.vim/bundle/YouCompleteMe
-    ./install.py --system-boost
+    ./install.py
+
+If the `python` executable is not present, or the default `python` is not the
+one that should be compiled against, specify the python interpreter explicitly:
+
+    python3 install.py --clang-completer
 
 The following additional language support options are available: